This invention relates to a magnet for use in a nuclear spin tomograph and more particularly to such a magnet having an opening which extends in a longitudinal direction and forms a cavity for receiving a patient, the magnet generating in the cavity an approximately homogeneous magnetic field which is perpendicular to the longitudinal axis of the magnet.
Such a magnet is disclosed in published PCT patent application WO-Al No. 84/01 226. The disclosed magnet generates a homogeneous magnetic field, wherein the relative inhomogeneity is 5.times.10.sup.-4. Outside the magnet, only a small stray field is generated, the major part of which is in the region of the end faces of the magnet. The magnet of the PCT application comprises approximately circular rings arranged one behind the other in the longitudinal direction. The rings have different inside diameters increasing towards the center of the magnet, whereby the homogeneity of the magnetic field in the interior is increased and a smaller overall length is achieved than with a magnet having a constant inside diameter. Each ring consists of several similar trapezoidal magnetized segments so that the cross sections of the opening of the magnet and the outer contour thereof are equilateral polygons, thereby giving the magnet approximately the form of a hollow cylinder. During the assembly of the magnet, gaps between the segments must be maintained for adjusting the positions of the segments. Accordingly, the segments of a ring do not engage each other. Because of these gaps, however, the intensity of the magnetic field in the cavity or opening of the magnet is not as great as the field generated in the case of a gapless cylindrical magnet made of permanent magnet material. A magnetic potential drop occurs in the gaps between the segments. In addition, the undesirable stray field due to the gaps is greater in the vicinity of the cylinder surface.
The manufacture of the above-described magnet is expensive in part because the procedures of magnetizing the segments and assembling them into a ring are complicated. The segments each consist of a permanent magnet material with a preferred orientation. Each segment is magnetized in the preferred orientation before the segments are joined together in a ring. The preferred orientation in each segment is different and depends on the location of the center of the segment upon assembly of the magnet. For this reason every segment must be magnetized separately.
Upon magnetization, each segment is held for assembly by a frame which has a fine adjustment screw by which the final position of the segment can be adjusted. During assembly, all frames with their respective segments are brought into their end positions by means of special guides. In the process, large magnetic forces between the segments occur which must be intercepted by and transmitted through the guides. The magnetic fields of the segments also influence each other during the motions of the assembly process, with the result that the magnetic field may not be sufficiently homogeneous in the patient receiving cavity. To correct the magnetic field in the cavity, the positions of all segments are varied by means of the fine adjustment screws. For each segment, several measurements and correction calculations by a data processing installation are necessary. After these corrections have been made, the segments are fixed in their positions and the frames are removed. In this manner one of the rings arranged in tandem is completed. Subsequently this ring is placed in a support frame. This procedure is repeated for each following ring and the positions of the rings in the support frame are fixed by further measurements and calculations.
An object of the present invention is to provide an improved magnet of permanent magnet material with a homogeneous field of the above-described type.
Another object of the present invention is to provide such a magnet which is simply manufactured.